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Design of Gravel Banks - A Way to Avoid Jack-Up Spudcan Punch-ThroughType of FailureLindita Kellezi, GEO-Danish Geotechnical Institute; Henrik Stadsgaard, Maersk Drilling

Copyright 2012, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in H ouston, Texas, USA, 30 April3 May 2012.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abst ractFor the installation of a three-legged jack-up rig at an oil field / drilling location in the North Sea conventional and finiteelement spudcan penetration analyses and engineering assessments are carried out. As the location, based on the preliminaryassessments, indicated critical soil conditions with regard to rig installation, an improvement / increase in the seabed bearingcapacity was aimed by designing / constructing gravel banks on the seabed, one at each spudcan location, respectively, as away to avoid jack-up foundation punch through type of failure.

The soil data at the location included three boreholes with sampling and laboratory testing combined with piezocone penetration tests (PCPTs) one at each spudcan centre, respectively. Furthermore, a site survey was available covering the areaof interest. Based on all the available soil data design lower / upper bound soil profiles applicable to penetration analyses ateach spudcan location for virgin seabed conditions were assessed first. Those profiles and the results from the different,conventional and finite element analyses carried out together with a validation study are presented. Based on them the decision

for modifying the seabed by constructing gravel banks was taken.

The gravel banks were designed based on the Plaxis finite element modelling applying lower bound soil parameters calculatingfinal heights of 4.0 m, 2.5 m, 2.0 m for Port, Starboard and Bow legs, respectively. The top diameter and the slope of the banksare chosen to be the same, 50 m and 1:2.5, respectively. Except the small strain (plastic) and updated mesh (UM) analyses anattempt was made to adjust the conventional solution for multilayered soil conditions in comparison to finite element results.

For the new conditions, the modified seabed by construction of the gravel banks, sufficient foundation bearing capacities andno risks for punch-through / rapid penetrations were calculated. The spudcan-gravel bank-soil interaction systems able tosupport the maximum preloads for each leg determined based on the environmental load analyses. The gravel banks wereconstructed and short after that the jack-up rig was successfully installed measuring shallow spudcan penetrations as predicted.

Introduction

Prediction of the jack-up spudcan penetrations is an important issue in the process of rig installation. Unexpected sudden andrapid penetrations can be of major risk for the stability and equilibrium of the jack-up structures. Classical conventionalsolutions (Hansen 1970), which are applications of bearing capacity equations for homogeneous soil conditions and modified

procedures for layered soil profiles (Jacobsen et al., 1977), (Hanna & Meyerhof 1980) are normally used for the spudcan penetration prediction. However, for layered soil condition the modified conventional procedures are not always sufficientlyaccurate and realistic comparable to field observations.

As analytical procedures have several limitations, alternative analyses based on numerical modeling are investigated and presented through the years. Applications of those methods mainly based on the finite element analyses in comparison toanalytical solutions for the conventional, non-skirted spudcans are among others elaborated in (Kellezi & Strmann 2003),(Kellezi et al., 2005), (Kellezi & Kudsk 2009).

A case history related to a jack-up installation in the North Sea is considered in the following. As part of the assessment, due to

multilayered / critical soil conditions conventional and finite element analyses of the spudcan penetrations for virgin seabed

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are carried out. As the results of the analyses indicated risk for punch-through / rapid penetration for the calculated maximum preloads at each leg, an improvement / increase in the seabed bearing capacity was aimed by designing gravel banks, one ateach spudcan location, respectively.

The concept of the application of the gravel banks as a way to avoid punch-through risk by locally increase the thickness of theexisting seabed sand layer is previously investigated and applied in other locations for a jack-up rig with skirted spudcans(Kellezi et al., 2007), (Kellezi et al., 2008). The necessity of the gravel banks at the current location for the installation of the

jack-up with conventional spudcans, the way they are designed and their benefit in fulfilling the required safeties areelaborated in the following sections.

Spudcan Dimensions and LoadsThe spudcans of the current rig have the equivalent radius of 8.9 m and the full contact area of 249 m 2. Distance from spudcan

base (full contact) to spudcan tip is 1.6 m. The tip has a minimum radius of 0.75 m and a maximum radius of 1.55 m. Thecentral distance between the spudcans is 62 m. The spudcan geometry is given in Figure 1. The initial load is about 6824 tons /leg and the maximum preload about 15700 tons / leg, giving a maximum spudcan pressure equal to 619 kN / m 2.

Figure 1: Spudcan geometry

Jack-up LocationThe position / heading of the jack-up is north-west and such that the spudcan centres nearly coincide with the location of theavailable boreholes / PCPTs carried out during a site investigation prior to the rig installation. The water depth at the site isabout 71.5 m Lowest Astronomical Tide (LAT). The seabed at the jack-up location is essentially flat deepening gently to thesoutheast. The mapped seabed is dominantly composed on fine to very fine sand. Seabed features do not imply any concernsregarding the rig installation.

Seismic Survey and Shallow SoilsA site survey has been carried out at the location. The survey with pinger and sparker covered a wider area encompassingseveral jack-up locations. Shallow lithology at the location comprises a thin veneer of Holocene sand underlined by Base Forthformation (medium dense to dense sand). Below this lie deposits of Coal Pit Formation comprising stiff to very stiff clays,silty sands and interlaminated clays and silty sands.

Geotechnical InvestigationThe geotechnical investigation at the location included three boreholes (F1, F2, F3) with alternating sampling and PCPTs atrespective spudcan positions. The boreholes show thin veneer of loose to medium dense sand propagating from the seabed to(0.3 0.5) m in depth, underlined by medium dense to dense sands to depths of (4.1 6.2) m. Below the sand, firm to stiffclay deposits are found to the end of the boreholes, with an exception at borehole / PCPT F3, where a layer of medium densesand is found at (11.5 19.2) m below seabed. Thin laminate / pockets of clay / silt / sand have been found at all the three

boreholes / PCPTs. The soil stratification in the boreholes reflects the shallow soils prediction given by the seismic survey,

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although, the Base Forth formation seems to be somewhat deeper.

The PCPTs show cone resistance q c of up to 20 MPa in surficial sands and to about 30 MPa in the deeper sand depositencountered at borehole PCPT F3. In clay layers, the cone resistance q c varies, and excluding the peaks caused by laminationor other inclusions, it is generally in the interval of (0 - 5) MPa. Strength tests on clay are carried out on the extracted samples.The undrained shear strength tests are carried out using pocket penetrometer, torvane, and unconsolidated undrained (UU)triaxial tests. The laboratory tests show good match with the PCPT results for the adopted cone factor N kt = (15 20).

Design Lower / Upper Bound Soil Profiles, Virgin SeabedBased on all the soil data, design lower / upper bound soil profiles for virgin seabed conditions are assessed applicable to

penetration analyses at each spudcan location. Some of the interpreted lower bound undrained shear strength profiles are givenin Figures (2-4) where a lower bound (blue line) and an alternative lower bound (red line) are shown.

Figure 2: Undrained shear strength and fric tion angl e at the Borehole / PCPT (Port Leg)

The upper / lower bound undrained shear strength c u, for the clay layers has been determined from the results of the laboratorytests (mainly UU triaxial tests) and net cone resistance, q net, using correlation factor N kt = (1520). This would fit with an

average equivalent isotropic shear strength, being the average of the active, direct shear and passive shear strength. The way

Alternative LowerBound

Lower Bound

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how the designed undrained shear strength profile follows the variation of q net / N kt depends also on the experience of thedesign engineer. As the current analyses are based on characteristic soil parameters (no material coefficients applied, just asfor a common spudcan penetration prediction following (SNAME 2002), a conservatively selected lower bound profile (bluelines in Figures (2-4) are chosen for the current assessments.

The angles of internal friction for the seabed sand layers are derived from the results of the PCPTs as peak values and criticalstate values. Critical state friction angles equal to 30 are applied as lower bound strength for the sand layers in the currentanalyses as shown in Figures (2-4).

Figure 3: Undrained s hear strength at the Borehol e / PCPT (Starboard Leg)

Relative density of the sand, Dr, is interpreted first from the PCPT data. The peak angle of internal friction is then based on therelative density. The values of the angle of internal friction utilized for the following analyses are re-assessed by reducing the

peak strengths or deriving the critical state angles as a difference of the peak and dilatancy values. This is because the angle offrictions depends on the volumetric changes related to the stress level. On high stress levels, dense sands dilate less than onlow stress levels. However for loose sands the dilation is smaller, reducing to zero for high stress levels.

The friction angles measured in the laboratory testing are the peak angles obtained while the sand is free to contract / dilate.

However, to account for the fact that in order to mobilize failure into the clay, large strains within the sand may result leading


Lower Bound

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to a drop in the friction angle. Therefore, the highest values that are applied in the current bearing capacity computations arehowever the critical state friction angles, i.e. the angles that correspond to the failure associated with no volumetric changes.The difference between the peak angles, and the critical state angles, represent the angles of dilatancy or a fraction (0.8) of theangles of dilatancy - depending on the type of the test, i.e. for triaxial or plane strain respectively. This results with roughly (2

5) difference between the two test types that are commonly taken for assessments of lower and upper bound failurestrengths.

Figure 4: Undrained sh ear strength at t he Borehol e / PCPT (Bow Leg)

Design Lower / Upper Bound Soil Profil es, Seabed Modifi ed by Gravel BanksBased on the bearing capacity analyses for virgin seabed conditions, which are discussed in the following, gravel banks werefound necessary to be constructed at the spudcan locations in order to increase the lower bound capacity to punch through /rapid penetrations.

The gravel banks are designed based on the lower bound soil parameters and applying conventional solutions for infiniteextended banks and finite element analyses for a prechosen geometry of the banks. After several analyses gravel banks withheights 4.0 m, 2.5 m, 2.0 m, were designed for Port, Starboard and Bow legs, respectively. The top diameter and the slopewere chosen to be the same, 50 m and 1:2.5, respectively.

Lower Bound


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Considering the above heights and assuming infinite extension of the banks in the horizontal plane, the design lower / upper bound soil profiles at each borehole / PCPT or spudcan location, after the construction of the gravel banks are modifiedcompared to the virgin seabed as shown in Tables (1-3). The upper bound parameters are only used to get an impression of theexpected maximum capacities at the leg locations.

For the gravel / rockfill material an angle of internal friction of 40 was chosen based on a conservative interpretation of thedata for the stress dependency of the angle of friction for such material. The actual gravel material was expected to be looselycompacted, but relatively uniformly graded material. However, it consists of high strength particles meaning that the particleinterlocking is not easy destroyed by crushing. Hence, the chosen strength was considered realistic. The deformation modulefor the gravel was chosen E=35000 kPa.

Soil Type Depth of Layer(m)Unit Weight

' (kN/m 3)Undrained ShearStrength c u (kPa)

Angle of InternalFriction ( )

GRAVEL 0.0 4.0 9.5 - 40 / 45SAND, medium dense to dense 4.0 8.1 9.5 - 30 / 35CLAY, firm to stiff 8.1 17.5 8.5 (80-60) / (105-80) -CLAY, firm to stiff 17.5 28.5 8.5 80 / 105 -CLAY, firm to stiff 28.5 33.0 8.5 (125-80) / (165-105) -CLAY, firm to stiff 33.0 34.0 8.5 (125 / (165) -

CLAY, firm to stiff 34.0 38.5. 8.5 (80-60) / (105-80)CLAY, stiff to hard 38.5 44.0 8.5 (60-210) / (80-280) -Table 1 Design l ower / upper bo und so il p rofil e, Borehole / PCPT F1, gravel bank h eight=4.0 m, (Port Leg)




GRAVEL 0.0 2.5 9.5 - 40 / 45SAND, medium dense to dense 2.5 8.7 9.5 - 30 / 35CLAY, firm to stiff 8.7 9.0 8.5 110 / 145 -CLAY, firm to stiff 9.0 18.0 8.5 (80-45) / (105-60) -CLAY, firm to stiff 18.0 31.5 8.5 (70-75) / (90-100) -CLAY, firm to stiff 31.5 32.8 8.5 (75-120) / (100-160) -

Table 2 Design l ower / upper bo und s oil p rofi le, Borehole / PCPT F2, gravel bank height=2.5 m, (Starboard Leg)




GRAVEL 0.0 2.0 9.5 - 40 / 45SAND, medium dense to dense 2.0 6.5 9.5 - 30 / 35CLAY, firm to stiff 6.5 13.5 8.5 (80-70) / (105-90) -SAND, medium dense to dense 13.5 21.2 9.5 - 30 / 35CLAY, firm to stiff 21.2 25.0 8.5 100 / 130 -CLAY, firm to stiff 25.0 28.0 8.5 75 / 100 -CLAY, firm to stiff 28.0 40.9 8.5 105 / 140 -

Table 3: Design lower / upper bound s oil pr ofil e, Borehole / PCPT F3, gravel bank height= 2.0 m, (Bow Leg)

Conventional Analyses for Virgin and Modified SeabedConventional or classical bearing capacity analyses are often based on limit equilibrium methods. Limit equilibrium is a stateof equilibrium corresponding to a failure criterion that defines the strength of the soil. Failure is usually defined as a statewhen the strength is fully mobilized along the entire failure surface. The penetration analyses follow the guidelines given in(SNAME 2002). The calculations are based on design soil parameters with partial coefficients, m = 1.0. In the calculations theloads are considered as purely static.

To define footing penetration versus load, assessment of the static bearing capacity of the spudcan at various depths is carriedout. The bearing capacity is calculated based on (Hansen 1970) theory and in-house program developed from the experiencewith spudcan penetration predictions. The spudcan is simplified to a circular footing having a flat bottom. The effect of theactual spudcan shape is taken into account. Squeezing of the clay layer underlying the sand during the footing penetration isconsidered and implemented. Soil backflow is included in the analyses.

The punch through mechanism was analysed based on the load spreading factor (SF) according to (SNAME 2002), (Jacobsenet al., 1977) and authors experience and by the punching mechanism or the method described by (Hanna & Meyerhof 1980)utilizing the coefficients given from Figure 5 derived from the same authors.

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During spudcan penetration through the sand layers a sand plug is assumed to develop and lead the spudcan penetrationthrough clay. This is confirmed by the finite element analyses. The conventional analysis utilized the thickness of the plugestimated with respect to finite element results. Punching failure is investigated by varying load spread factor (SF), whichdepends on the ratio between the spudcan diameter and the sand thickness, the average strength of the sand layers etc. SFadopted for the analysis has been considered also in relation to the results of subsequent finite element analyses.

For the modified seabed conditions the soil profile is constructed by adding a layer of gravel over the seabed with thecorresponding height depending on the spudcan location. In the analyses the seabed level is assumed at the top of the banks.

Figure 5 Coefficients of punching shear resistance under vertical load

Finite Element Analyses for Virgin and Modified SeabedFinite element modeling of the spudcan penetration is carried out with (Plaxis 2008) finite element software. Due to themultilayered soil profile finite element modelling of the large spudcan penetrations is complex. When using finite elementmethods both the deformations and the failure criterion are defined through a material model. In this way deformationcorresponding to a certain level of soil strength utilization can be chosen. Hence, small deformations or large deformationstheories can apply. Considering the program limitations, the following assumptions / simplifications are made in building thefinite element model in Plaxis:

Due to symmetrical vertical loading / penetration, axisymmetric modelling of the spudcan-soil interaction system iscarried out taking the three-dimensional effect into account.

Sand layers are modelled in drained condition using Mohr-Coulomb constitutive model. Clay layers are modelled inundrained conditions using Mohr Coulomb or Tresca soil model, assuming ideal elastic-perfectly plastic soil

behaviour. Isotropic shear strength and tension cut-off is applied. The dilatancy angle was taken as = -30. Soilsoftening and rate effects can be taken into account in advanced soil models, however, not available in the finiteelement software.

The initial geostatic stresses are calculated for virgin soil first based on the K 0-procedure. In line with theconventional analyses, finite element analyses utilize effective unit weights for the seabed soil layers. Hence, thewater table is placed at the bottom boundary of the model.

Mesh has been generated using cubic (15-noded) triangular finite elements, which results with lower bearing capacitythan when using quadratic (6-noded) triangular elements.

The spudcan is modelled as a weightless rigid body. Interface elements are used between the structure and the soil with a strength reduction factor of 0.7.

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The deformation parameters (E module) for sand layers are calculated based on the PCPT data, and for the clay layerson the undrained shear strength using the correlation E = 200c u. These assessments of the soil stiffnesss aresupported from the laboratory testing. However, for the current assessment the strength parameters play the main role.

The analysis starts with spudcan in-place with full base contact with the seabed or initial penetration of 1.6 m. Vertical displacement is applied at the full base of the spudcan and the reaction force is computed and recorded. Soil backflow generally is not / cannot be modelled in the Plaxis finite element analyses. Therefore, corrections of the

bearing capacity are made when plotting the Plaxis curves at the depth where backflow is expected to occur. In thecurrent analyses, the finite element results are considered only for the depths where the backflow / inflow are notexpected to affect the results.

For the modelling of the spudcan-gravel bank-soil interaction, the gravel bank is constructed first giving changes inthe initial stress conditions and excess pore pressure at the underlying clay layers. The dissipation of the excess pore

pressure in time is conservatively not considered in the analyses.

Figure 6: Incremental shear strain (failu re pattern) at 3 m spudcan penetration (Port L eg)Left small strain (Plastic) analysis; Right large deformation (UM) analysis

Figure 7: Shear strain at 3 m spudcan penetratio n (Starboard Leg)Left small strain (Plastic) analysis; Right large deformation (UM) analysis

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Figure 8: Shear str ain at 3 m spudcan penetration (Bow Leg)Left small strain (Plastic) analysis; Right large deformation (UM) analysis

At first, mesh sensitivity analyses have been carried out and the optimal mesh with respect to element size (inversely proportional to the computational time) and obtained accuracy has been chosen for the final analyses. Computations, using both small strains (Plastic) and large deformation or updated mesh techniques are performed. Some results for the modifiedseabed (including the construction of the gravel banks) are given in Figures (6-8).

Plaxis Small Strains and Updated Mesh AnalysesSome elaborations on the Plaxis small strains plastic analysis and plastic updated mesh analyses are given in this section.Generally in finite element analysis, the influence of the geometry change of the mesh on the equilibrium conditions isneglected. This is usually a good approximation when the deformations are relatively small as is the case for most engineeringstructures. However, there are circumstances under which it is necessary to take this influence into account.

Typical applications where updated mesh analyses may be necessary include the analysis of reinforced soil structures, theanalysis of large offshore footing collapse problems and the study of problems where soils are soft and large deformationsoccur. When large deformation theory is included in the finite element program, some special features are considered like:

Additional terms in the structure stiffness matrix are included to model the effects of large structural distortions on thefinite element equations.

A procedure to model correctly the stress changes that occur when finite material rotations happen is implemented.This particular feature of large displacement theory is usually dealt with by adopting a definition of stress rate thatincludes rotation rate terms.

The finite element mesh is updated as the calculation proceeds. This is done automatically within Plaxis when the

updated mesh option is selected. These calculation procedures are in fact based on an approach known as an UpdatedLagrangian formulation.

It is not possible to give simple guidelines that may be used to indicate when an updated mesh analysis is necessary and whena conventional analysis is sufficient. If the geometry changes are large (on a real scale!) then significant importance ofgeometric effects might be suspected. In this case the calculations are repeated using the updated mesh option.

Validation Study Conventional and Finite Element Plaxis AnalysesSeveral validations of the Plaxis analyses applicable to offshore foundations are previously performed by the authors.However, due to the differences found in the calculated spudcan bearing capacities based on the conventional (load spreadingfactor or punching mechanism) and Plaxis analyses for the current multi-layered virgin seabed, additional validations werecarried out.

In this validation study the accuracy of the conventional analysis is at first investigated assuming a theoretical profile (Test

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Site) consisting of a simply two-layer system: sand over clay. The virgin soil profile interpreted from the borehole / PCPT F1is simplified, (however, keeping most of the features), by excluding the strength reduction with depth for the clay layerunderlying the sand as shown in Figure 2 (blue line).

The results for the Test Site with sand friction angles of 35

and 40

and clay with constant c u = 80 kPa are given in Figure 9left. It is observed that conventional analyses utilizing SF 1:4 & 1:3 fit well with Plaxis updated mesh (UM) results at depth of2.6 m (spudcan tip to base, plus spudcan height over the maximum area), while Plaxis small strain (plastic) analyses giveunrealistically low capacities.

The same Test Site but with friction angle of 30 for the seabed sand layer is carried out afterwards. The conventionalanalysis utilizing SF 1:5 overestimate the peak bearing capacity in comparison to Plaxis updated mesh (UM) analyses, andeven more Plaxis small strain (plastic) analyses. The results of the method using load spreading factor (SF) are comparablewith the Plaxis updated mesh (UM) results only for SF reduced to 1:12 as shown in Figure 9 right. This is supported by thestudy of (Jacobsen et al., 1977).

The above analyses for the Test Site are conducted for varying sand friction angles and keeping the underlying clay strengthconstant. As a result, it is concluded that when the strength of the top sand layer is not very large in comparison to the strengthof the underlying clay layer, the conventional methods utilizing SF or punching mechanism may not comply well with thefinite element results giving lower bearing capacity or giving more a risk for rapid penetration than punch through.

Conclusions of the validation are as in the following:

Plaxis updated mesh (UM) analyses result with larger bearing capacities than Plaxis small strain (plastic) analysis. Application of the Plaxis updated mesh (UM) analysis on the spudcan penetration is more or less validated. Plaxis small strain (plastic) analysis is evaluated to be conservative and non-complying with the conventional

methods. However, for clay soils where softening behaviour is indicated from the laboratory tests (reduction of the undrained

shear strength with increasing strain), small strain analyses can be used in absence of a softening soil model in Plaxis. When the relative difference in the soil strengths for sand over clay type of soil profile is small, conventional method

applied for strong over soft layer type of failure, needs to be updated by reducing the SF with respect to the results ofPlaxis updated mesh (UM) analysis or alternatively re-assessing the coefficients given in Figure 5.

Figure 9: Conventional and FE results of spu dcan penetration on th e Test SiteLeft medium to very dense sand; Right loose to medium d ense sand

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1:2.5, respectively, beside the small strain (plastic) and updated mesh (UM) analyses an attempt is made to derive theconventional solution, which compare well with the finite element results. These solutions are given in Figures (10-12).

At the borehole / PCPT F2 where the strength decrease with depth is more pronounced as noticed from the blue line in Figure3, there is more discrepancy between those methods. The finite element results in the form of incremental shear strain orfailure patterns are shown in Figures (6-8) for both types of the finite element analyses.

Conclusions and RecommendationsThe engineering assessment for the installation of a jack-up rig, whos spudcan geometry is given in Figure 1, at a drillinglocation in the North Sea is carried out. Based on the available soil data design lower / upper bound soil profiles for virginseabed conditions, applicable to penetration analyses at each spudcan location are assessed first. Those soil profiles are

presented in Figures (2-4) and the results of the different, conventional and finite element, small strain (plastic) and updatedmesh (UM) analyses are given in Figures (10-12) for each spudcan location.

Figure 11: Conventi onal and FE results of spud can penetration on vir gin and modi fied (by gravel banks) seabed (Starboard Leg)

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Due to the observed discrepancies between conventional and Plaxis analyses results for the virgin seabed design soil profiles(lower bound), a validation study was performed assuming a Test Site consisting of two-layer system, sand over clay. Theanalyses for the Test Site are conducted for varying sand friction angles and keeping the underlying clay strength constant.

Figure 12: Conventional and FE results of spudcan penetration on virgin and modified (by gravel banks) seabed (Bow Leg)

The results are given in Figure 9. Based on them it is concluded that when the strength of the top sand layer is not very largein comparison to the strength of the underlying clay layer, the conventional methods utilizing load spreading factor (SF) or

punching mechanism applying the coefficients given from the graphs in Figure 5 may not comply well with the finite elementresults, which give lower bearing capacity or more a risk for rapid penetration than for punch through.

Both Plaxis analyses, small strain (plastic) and plastic updated mesh (UM) are investigated. The results with Plaxis UM seemto compare reasonably well with the conventional methods, while Plaxis small strain (plastic) analyses result with smallercapacities. Based on the validation study the correction for SF from 1:5 to 1:12 is implemented in the lower boundconventional analyses for all three virgin soil profiles at the locations as given in Figures (10-12). If upper bound strength is

applied to the sand, which based on the calculation of peak and critical state friction angles could be the case, conventional

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method results with SF (1.5-1:4) and Plaxis UM compare relatively well, while plastic analyses give reduced capacities as can be observed in Figure 9.

The application of SF 1:12 seem to work well for the current lower bound conditions. However, as the SF value depends onthe geometry of the soil profile, the relative strength of the layers, the strength variation within the layers etc., suchconventional analyses need to be confirmed by alternative methods such as Plaxis UM method or by applying other finiteelement software..

For the modified seabed by the construction of the gravel banks as designed based on the Plaxis finite element modellinghaving heights of 4.0 m, 2.5 m, 2.0 m for Port, Starboard and Bow legs, respectively and the top diameter and the slope of the

banks the same, 50 m and 1:2.5, respectively the results in the form of the failure pattern / figure are given in Figures (6-8) andin the form of bearing capacity or penetration curves in Figures (10-12).

The differences in the bearing capacity given from the small strain (plastic) analyses and plastic updated mesh (UM) analysescan be explained with the differences in the shape and size of the failure patterns given in Figures (6-8). In these figures it iseasy to see how the radial extent of the gravel bank influences on the shape of the failure mechanism. The gravel bank has ahigh capacity and will push the failure line out radially in the underlying clay, rather than going through the gravel. At acertain distance the failure mechanism will find the lowest bearing capacity by going through the bank, which means a largerradius of the bank, has no advantages. The necessary extensions of the gravel banks are determined by comparing the resultswith infinite extended banks.

Except the small strain (plastic) and updated mesh (UM) analyses an attempt was made to adjust also in this case conventionalsolution for multilayered soil conditions in comparison to finite element results. At the borehole / PCPT F2 where the shearstrength decrease with depth is stronger as indicated in Figure 3, there is more discrepancy between those methods.

For the modified seabed by the gravel banks as designed no risk for punch through / rapid penetration was expected at thecalculated preload levels. From the results given in Figures (10-12) sufficient bearing capacities at each leg was interpreted incomparizon to the preloads to be applied.

Independent spudcan-soil interaction and after that spudcan-gravel bank-soil interactions analyses are carried out for thecurrent project. This means that structure-foundation interaction has been neglected being on the safe side. Such interactionwas taken into account from ( Kellezi et al., 2007) and (Kellezi et al., 2008) for the world largest platform installed at another locationoffshore Norway.

Construction of t he Gravel Banks, Jack-Up Rig Installing and Field ObservationsThe gravel banks were built-up / constructed as shown in the plan view drawing given in Figure 13.

Figure 13 Plan drawing of th e gravel bank cones as constr ucted at the location

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